Paint coatings are surface protective coatings applied to substrates, dried to form continuous films for decorative purposes as well as to protect the substrate. Consumer paint coatings are air-drying aqueous coatings applied primarily to architectural interior or exterior surfaces, where the coatings are sufficiently fluid to flow out, form a continuous paint film, and dry at ambient temperatures to protect the substrate surface. A paint coating ordinarily comprises an organic polymeric binder, pigments, and various paint additives. In dried paint films, the polymeric binder functions as a binder for the pigments and provides adhesion of the dried paint film to the substrate. The pigments may be organic or inorganic and functionally contribute to opacity and color in addition to durability and hardness, although some paint coatings contain little or no opacifying pigments and are described as clear coatings. The manufacture of paint coatings involves the preparation of a polymeric binder, mixing of component materials, grinding of pigments in a dispersant medium, and thinning to commercial standards.
Latex paints for the consumer market ordinarily are based on polymeric binders prepared by emulsion polymerization of ethylenic monomers. A typical consumer latex paint binder contains a vinyl acetate copolymer consisting of polymerized vinyl acetate (80%) and butyl acrylate (20%). The hardness of the latex polymer must be balanced to permit drying and film formation at low application temperatures, which requires soft polymer units, while at the same time the polymer must be hard enough in the final film to provide resistance properties which requires hard polymer units. This is conventionally accomplished by designing a latex polymer with a moderately elevated Tg (glass transition temperature) but then lowering the Tg temporarily with a volatile coalescing solvent. Coalescing solvents function to externally and temporarily plasticize the latex polymer for time sufficient to develop film formation, but then diffuse out of the coalesced film after film formation, which permits film formation and subsequent development of the desired film hardness by the volatilization of the coalescent. Internal plasticization is based on coreaction of soft monomers with hard monomers to form a polymeric copolymer binder, such as 80/20 vinyl acetate/butyl acrylate, to obtain the desired film forming characteristics. If a lower Tg copolymer is used without a coalescing solvent, higher levels of soft comonomer are required leading to lower Tg polymer, and, hence, the final dried film would be undesirably soft, excessively tacky, readily stain, and readily pick up dirt.
A significant source of residual odor in latex consumer paints is directly due to the coalescing solvent. Coalescing solvents are typically linear (or slightly branched) glycol ethers and esters of 7 to 12 carbon atoms in length, which have boiling points typically above 200.degree. C., and solubility parameters appropriate for the latex of interest. One typical coalescing solvent ordinarily contained in commercial latex paints is 2,2,4-trimethylpentanediol monoisobutyrare (Texanol.RTM. Eastman Chemical Co.). The odor associated with the gradual volatilization of this solvent is considered objectionable by consumers. Quite often the odor lingers for days or weeks after the paint is applied and dried. All useful coalescing solvents are volatile and have similar objectionable characteristics. An additional deficiency in conventional exterior latex paints is the decline in crack resistance of the dried paint film approximately proportional to the evaporation of the coalescing solvent. While better coalescing solvents have a retention time of about one year in dried paint films, cracking starts to progressively appear after one year in dried paint films. Hence, the elimination of coalescing solvents and attendant objectionable odors, along with air pollution caused by volatile organic compounds (VOC), and film cracking deficiencies represent both a technical and marketing advance in the state of the art of consumer latex paints.
In polymer technologies unrelated to air-dry vinyl acetate latex paints, preformed polymers have been dispersed into monomers and emulsified in water, whereupon the monomers are then polymerized, such as disclosed in U.S. Pat. No. 4,373,054 pertaining to cathodic electrocoating, or in U.S. Pat. No. 4,313,073 pertaining to alkyd prepolymers; U.S. Pat. No. 4,588,757 pertaining to laminating adhesives, or in U.S. Pat. Nos. 3,953,386 and 4,011,388 pertaining to aqueous emulsion blends containing cellulosic ester/acrylic polymers.
It now has been found that certain non-volatile softening oligomeric modifiers compatible with a polyvinyl acetate matrix polymeric binder in a consumer latex paint can be retained in the dried paint film permanently. Softening oligomers of this invention can be incorporated into the paint where the oligomer will be retained permanently in the final paint film. Hence, the paint will not generate an odor while drying nor emit a residual odor from the dried paint film or otherwise emit VOC's. The softening oligomeric modifiers of this invention externally modify the polyvinyl acetate and are not coreacted with the polyvinyl acetate polymeric binder. The softening oligomeric modifiers appear to function by a chain-spacing mechanism to soften the polyvinyl acetate polymers whereby the oligomeric modifiers provide low temperature film formation and tack-free films less prone to soiling at a given hardness and/or flexibility than ordinarily possible. A further advantage of this invention enable the use of essentially all hard polymer units of polyvinyl acetate without the need for internal plasticization (coreaction) with soft butyl acrylate polymeric units.
The present invention is based on a softening oligomeric modifiers having a low and narrow molecular weight range where the oligomer will not subsequently diffuse out of the matrix polyvinyl acetate polymer when properly dispersed into the polymeric binder phase. Preferred oligomers are non-volatile oligomers having a molecular weight between about 300 and 10,000. Lower molecular weight compounds tend to be volatile and cause excessive plasticizer migration while higher molecular weight polymers lose low temperature film-forming and softening effects, although molecular weights above 10,000 and in some instances up to 20,000 can be used with softer matrix polymers. According to the process of this invention, a compatible organic solution of oligomeric modifier in ethylenic monomer is subjected to high energy shear to prepare a sub-micron size organic phase dispersed into water. Subsequent polymerization of the micronized monomer droplets produces a softened modified latex very different from conventional emulsion or suspension polymerization polymers. The micro suspension polymerization is generally necessary with the oligomeric modifiers to accomplish the required sub-micron aqueous emulsification of the monomer containing the dissolved oligomeric modifier, since the oligomer will not readily diffuse during polymerization from particle to particle across the aqueous phase.
The principal advantage of this invention is the elimination of the odor and VOC associated with volatile coalescent solvents which are intentionally volatile and intended to migrate out of the dried paint film. An additional advantage pertains to dried paint films exhibiting superior toughness obtained through the use of a hard polyvinyl acetate matrix polymer balanced with the oligomeric modifier to accommodate softening through the external addition of softening modifier while retaining the desired dried film hardness. A further advantage pertains to lower net cost for both interior and exterior paints since high cost soft monomers can be avoided, volatile coalescing agents can be eliminated and binder volume can be increased by using a permanent non-volatile softening oligomer instead of a volatile coalescent. The resulting dried paint films exhibit a superior balance of hardness and flexibility while maintaining long term flexibility. These and other advantages of this invention will become more apparent by referring to the detailed description and illustrative examples.